Cancer is a complex multifactor and multistep process involving the coordinated expression and suppression of genes functioning as positive and negative regulators of oncogenesis (1-5). Direct cloning strategies, based on transfer of a dominant transforming or tumorigenic phenotype, have identified positive acting oncogenes (6-9). In contrast, the detection and cloning of genes that suppress the cancer phenotype have proven more difficult and elusive (10-15). A direct approach for isolating genes directly involved in regulating growth and differentiation involves subtraction hybridization between cDNA libraries constructed from actively growing cancer cells and cDNA libraries from cancer cells induced to lose proliferative capacity irreversibly and terminally differentiate (13,14). This experimental strategy has been applied to human melanoma cells, induced to terminally differentiate by treatment with recombinant human interferon β (IFN-β) and mezerein (MEZ), resulting in the cloning of novel melanoma differentiation-associated (mda) genes not previously described in DNA data bases (13,14). A direct role for specific mda genes in mediating growth and cell cycle control is apparent by the identification and cloning of mda-6 (13-16), which is identical to the ubiquitous inhibitor of cyclin-dependent kinases p21 (17). The importance of p21 in growth control is well documented and this gene has been independently isolated, as WAF-1, CIP-1, and SDI-1, by a number of laboratories using different approaches (18-20). These studies indicate that specific genes associated with proliferative control are induced and may contribute to the processes of growth arrest and terminal differentiation in human cancer cells.
The mda-7 gene was cloned from a differentiation inducer (IFN-β plus MEZ)-treated human melanoma (H0-1) subtracted library (13,14). The full-length mda-7 cDNA is 1718 nucleotides, and the major open reading frame encodes a novel protein of 206 aa with an Mr of 23.8 kDa (21). Previous studies indicate that mda-7 is induced as a function of growth arrest and induction of terminal differentiation in human melanoma cells (14,21). mda-7 expression also inversely correlates with melanoma progression—i.e., actively growing normal human melanocytes express more mda-7 than metastatic human melanoma cells (21). Moreover, mda-7 is growth inhibitory toward human melanoma cells in transient transfection assays and in stable transformed cells containing a dexamethasone (DEX)-inducible mda-7 gene (21). These studies indicate that mda-7 may contribute to the physiology of human melanocytes and melanomas, and this gene has growth suppressive properties when overexpressed in human melanoma cells.
The mda-7 gene was also described in the International Patent Cooperation Treaty Application No. PCT/US94/12160, international filing date, Oct. 24, 1994 with International Publication No. WO95/11986, the content of which is incorporated into this application by reference.
This invention reports that mda-7 is a potent growth suppressing gene in cancer cells of diverse origin, including breast, central nervous system, cervix, colon, prostate and connective tissue. An inhibition in colony formation occurs in cancer cells containing defects in their p53 and/or retinoblastoma (RB) genes or lacking p53 and RB expression. In contrast, expression of mda-7 in normal human mammary epithelial cells, human skin fibroblasts and rat embryo fibroblasts induces quantitatively less growth suppression than in cancer cells. When stably expressed in human cervical carcinoma (HeLa) and prostate carcinoma (DU-145) cells, mda-7 has a negative effect on growth and transformation-related properties. The effects of mda-7 on HeLa cells are reversible following abrogation of the MDA-7 protein by infection with a genetically modified Ad5 vector expressing an antisense mda-7 gene. These observations indicate that mda-7 is a novel growth suppressing gene with a wide range of inhibitory actions in human cancers manifesting different genetic defects.